HOW’S he doing? Quite well at 85. Former mayor Ed Koch never ran out of air when he incessantly asked that question of voters, but just in case, law firm Bryan Cave has invited a few friends to help Hizzoner blow out all 85 candles at a pre- birthday party at the St. Regis next week. The invite list in cludes Henry Kissinger, Bill and Hillary Clinton, Mayor Bloomberg, Tina Brown, Mario Cuomo, Pete Hamill, Al D’Amato and Archbishop Timothy Dolan.